1. Field of the Invention
This invention relates generally to a discharge machining apparatus for machining a workpiece by electrical discharge into desired dimensions and surface roughness, and more particularly to a discharge machining apparatus adapted to realize improvements in electrode consumption and in the surface quality of the machining surface.
2. Description of the Prior Art
In a discharge machining apparatus in which a workpiece is machined by electrical discharge caused by applying a d-c voltage across an electrode and the workpiece, the electrode is inevitably worn out so long as discharge is maintained. It has been experimentally confirmed that there is a relationship as shown in FIG. 5 among the period in which a discharge current flows as a d-c voltage is applied in the form of pulses, or the width of the discharge current pulse; the value of discharge current; and the amount of electrode consumption. In other words, if the discharge current value I.sub.P is constant, the lower the discharge current pulse width the higher becomes electrode consumption. And, if the discharge current pulse width is constant, the higher the discharge current value I.sub.P the higher becomes electrode consumption.
In a discharge machining apparatus of a conventional type, therefore, a discharge voltage pulse V and a discharge current pulse I are usually set as shown in FIG. 4. That is, discharge machining is performed by setting the discharge current value I.sub.P and the discharge current ON duration T.sub.ON at such values that electrode consumption can be minimized within a permissible range where energy can be maintained at a level required for discharge machining. Now, supplementary description will be made, referring to waveforms shown in FIG. 4. The waveform of the discharge voltage V steeply rises at the initial stage and then lowers stepwise in the later stages because a voltage application does not necessarily initiates discharge. S.sub.ON refers to the duration in which a voltage is applied, i.e., the ON duration of the discharge voltage pulse, while S.sub.OFF to the duration in which the voltage is not applied, i.e., the OFF duration of the discharge voltage pulse. T.sub.OFF denotes the duration from the time when the discharge current stops flowing to the time when the next voltage is applied.
With the prior art, however, electrode consumption can be reduced so long as discharge is performed properly. In the actual discharge machining, an abnormal phenomenon, such as an instantaneous interruption of discharge current or an electrical shortcircuiting between the electrode and the workpiece. often occurs. When any of such abnormal phenomena takes place, the prior art has the following problems.